Unix or Linux
1. Default Central inventory location present in the file /etc/oraInst.loc (AIX and Linux) or /var/opt/oracle/oraInst.loc (other platform)
2. If you don’t find then you can create the file with oraInst.loc please take care of capital or small letters Linux is case sensitivity.Example:
inventory_loc=/u01/oracle/oraInventory
inst_group=oinstall

2. If already exists then rename the old inventory and if not exists then create the default location as mentioned in above step.

5. For Example you need to set Oracle_home and Oracle_home_name as value to attached the Oracle Home with new inventory or existing inventory:
./runInstaller -silent -ignoreSysPrereqs -attachHome ORACLE_HOME="/u01/oracle/product/12.1.0" ORACLE_HOME_NAME="OraDb12c_home1"

Note: If you have more than one home then execute last step by changing oracle_home and oracle_home_name parameter